1. Field of the Invention
The invention relates to a sensing device, more particularly to a sensing device including a sensor layer of an insulator made from lanthanide-titanium oxide.
2. Description of the Related Art
Wu Min-Hsien, et. al.; “Structural properties and sensing performance of high-k Sm2O3 membrane-based electrolyte solution-insulator-semiconductor for pH and urea detection” Sensors and actuators, B: Chemical (ISSN 0925-4005), Volume 138, Issue 1, 24 Apr. 2009, pages 221-227, discloses a conventional sensing device for detecting the concentration of hydrogen ions (pH value) or substances in an electrolyte solution 100. Referring to FIG. 1, the conventional detecting system includes a semiconductor layer 111 of a field effect semiconductor having upper and lower surfaces, a lower electrode 112 formed on the lower surface of the semiconductor layer 111, a sensor layer 113 of the Sm2O3 membrane formed on the upper surface of the semiconductor layer 111, a protective cover layer 122 partially covering the sensor layer 113 and formed with a through-hole to expose a portion of the sensor layer 113 which is to be in contact with the electrolyte solution 100, and a reference electrode 13 disposed adjacent to the exposed portion of the sensor layer 113 for applying a potential to the exposed portion of the sensor layer 113. In operation, the sensing device is placed in contact with the electrolyte solution 100 with the reference electrode 13 extending into the electrolyte solution 100 and disposed adjacent to the through-hole in the protective cover layer 122, followed by applying different potentials to the reference electrode 13 for obtaining a capacitance-voltage characteristic curve (C-V curve) of the sensing device. The field effect semiconductor can be a p-type or an n-type semiconductor, and is capable of forming a field-induced inversion region of charges (not shown) when a voltage is applied across the reference electrode 13 and the lower electrode 112. The presence of the inversion region in the field effect semiconductor renders the sensing device to possess a characteristic C-V curve which varies with the amount of charges in the inversion region. The concentration of the ions in the electrolyte solution 100 can affect the potential at the sensor layer 113 applied by the reference electrode 13, which, in turn, affects the amount of the charges in the inversion region. As such, an increase or a decrease in the concentration of the ions causes shift of the C-V curve along a direction. Hence, through a measured C-V curve of the sensing device, the concentration of the ions in the electrolyte solution 100 can be determined.
Although the Sm2O3 membrane of the aforesaid conventional sensing device is capable of providing a stable C-V curve, there is still a need to enhance the detection sensitivity of the sensing de vice and to reduce the hysteresis voltage and the drift rate of the sensing device.